Selenium rectifier disc



Patented June 17, 1947 UNITED STATES PATENT OFFICE SELENIUM RECTIFIER nrsc Stanley S. Fry, North Chicago, Ill., assignor to Fansteel Metallurgical Corporation, North Chicago, 111., a corporation of New York No Drawing. Application November 11, 1944,

Serial No. 563,102

Claims. (Cl. 175-366) production of selenium rectifiers includes grit blasting the iron disc or plate and electroplating the disc with nickel. The purpose of the grit blasting is to present a roughened surface to the selenium and to thereby improve the adherence of the selenium to the carrier electrode. The disc is then thoroughly cleaned and a thin film of selenium is applied over the nickel layer.

The selenium film or layer may be formed by a variety of methods. The plate or disc may be heated to a temperature above the melting point of selenium, for example, to a temper.- ature of from 230 to 250 C., and the selenium in stick form may be rubbed across the heated plate in order to melt the selenium and form the desired film. Another method of application includes placing a measured quantity of powdered selenium or selenium in pellet form on approaching the melting point of selenium.

This'heat treatment is generally conducted at temperatures between about 200 C. and the melting point of selenium, for example, about 210 0., for a period of from fifteen minutes to several hours. During the combined heat treatment, a. layer of selenium is produced having a smooth surface of more or less uniform thickness and the selenium is transformed from its amorphous, non-conducting form into its gray,

. crystalline, conducting form.

The smooth surface of the selenium film is then treated to form an artificial blocking layer and a counter-electrode consisting of a relatively low melting point alloy is applied, as by spraying, over the selenium coating.

The final step in the manufacture of the rectifier plates consists of an electrical forming treatment. This treatment consists of subjecting the plate to either an alternating or direct current until a high resistance is developed in the reverse direction. This step may consist of I applying to the plate in the reverse direction a direct current voltage below about 15 volts or a pulsating direct current voltage starting with a low voltage of about 8 volts and gradually increasing the voltage to about 21 volts over a per od of several hours.

the heated plate and flowing themelted selenium over the surface. The melted selenium is usually spread over the heated plate by mechanical means, as with a glass rod. The selenium may also be-deposited on the carrier electrode from 1 Jacent plates, and the stack subjected to a moderate pressure. The stacks under pressure are then subjected to a relatively low temperature,

that is, at a. temperature below 150 'C., and

maintained at such temperatures for from one hour to four or five hours. Durin this stage of the heat treatment the selenium softens so that the pressureproduces a layer of selenium of relatively uniform thickness and having a smooth surface.

The stacks are withdrawn from the oven or heat treating furnace and the plates are removed from. the stack. The plates are then given a further heat treatment at a temperature In the use of an alternating current, it is necessary to include in the circuit a current limiting resistor because one-half cyclefiows in the forward direction of the rectifier plate. The voltage may be about 20 volts and the period required for forming is generally greater than that required when a direct current is employed because only one-half cycle of the current fiows in the reverse direction.

Light sensitive devices of the blocking layer class are produced in a' similar manner. A supporting electrode is provided with a thin film or layer of selenium in its gray, crystalline modification and an artificial blocking layer formed on the surface of the selenium. A second electrode is then applied over the. treated surface of the selenium. This electrode may consist of a light-permeable film of metal or a metal grid. .7 I

ture of the elements are dependent upon the type of unit being made and the class of service for which it is designed. For purposes of illustration, the production of selenium rectifier plates is described. I

A carrier electrod consisting of a grit blasted, nickel-Plated iron disc is provided with :a layer: or film of selenium in its gray, crystalline form by any of the aforementioned methods. The seleniumv may contain added materials toimprove th conductivity, as is wellknown to;.the art; The selenium surface is then exposed to the hot vapors resulting from the-heating of potassium thiocyanate toits decomposition temperature. The period of exposure of th selenium surface to the-vapors may vary from a few'seconds to severalminutes. I prefer to expose the plate to these vapors for from aboutfive secondsto about fifteen seconds.

The potassium thiocyanate may be placed in an open container and heated to the required temperature. The plates are exposed individually to the vapors by manually or mechanically-passing each plate over the open top of the container through the rising vapors. The plates may be placed upon an endless conveyorbelt which passes through a chamber having an atmosphere of the vapors.

The counter-electrode is then applied as by spraying a low melting point alloy over the treated selenium surface. The rectifier plate is finally subjected to any of the desired electrical forming processes known to the art.

One of the methods commonly employed in grading and in evalulating the quality of selenium rectifier plates is the measurementof the current passed by the plate in the reverse or blocking direction upon the application of a direct current voltage to the plate in the blocking direction. An ideal or theoretically perfectrectifier plate would have no leakage current flow under such condition.

In order to compare the current leakage of rectifier plates made in accordance with my inabout hour. This group of Plates was then divided into two series.

The discs of one series of plates were individually exposed for about 10 seconds to vapors formed by heating potassium thiocyanate to its decomposition temperature in an open vessel. The discs of the otherseries were used as standard or control plates.

The discs were subsequently, provided with a counterelectrode and subjected'to the' same electrical forming treatment.

A rectified direct current voltage of volts was applied in the reverse or blocking direction to theindividual discs of oth series of plates. The leakage current or the reverse current fiow in the standard or control plates of group A averaged approximately 50 milliamperes. The leakage current in the series of plates of group A made in accordance with the present invention averaged approximately 7 milliamperes. The plates of this group were also subjected to a rectified direct current voltage of 23 volts applied in the reverse or blocking direction tothe individual discs. The leakage current or the reverse current flow in the standard or control plates of group A averaged approximately 100 milliamperes. The leakage current in the series of group A plates made in accordance with my invention averaged approximately 10 milliamperes.

vention and rectifier plates made without the artificial blocking layer, a first group of nickelplated iron discs, havingan area of about 12 sq. cm., was provided with va layer of selenium in its gray, crystalline form. The discs were heated to a temperature of about. 240 C. and selenium in stick'form rubbed across the plate. Th selenium was'spread over the surface of the heated discs with a glass rod. The selenium employed in coating this group of discs, group A, was a normal grade selenium for rectifier manufacture. The plates were then stacked with mica discs covering the selenium layer and the stack placed under pressure, The stack was heated to a temperature below 150 C for about 2 hours. The pressure was then removed and the plates heated to about 210 C. and maintained at this temperature for vA second group of plates, group B, having an area of about 12 sq. cm., was prepared in the same manner. This group of plates, however, was pro-v vided with a layer of a lower grade selenium to which had been added about 0.1% iron cesium bromide so as to increase the conductivity of the selenium; One seriesoi'nplatesoi'this group was used as astandard or control. The selenium surface of the other series of group 3 plates was exposed for about 10 seconds to the action of vapors produced by heating potassium thiocyanate to its decomposition temperature in an open vesseL' A rectified direct current voltage of 20 volts was applied in the reverse'or blocking direction to the individual plates or discs of both series. The leakage current in the case of the control plates averaged approximately milliamperes. The leakage current in the case oi" the plates made in accordanc with this invention averaged approximately20 milliamperes.

The rectification ratio, another measure or indication of the quality of rectifierfplates, for an ideal or theoretically perfect rectifier plate would be 0. This ratio is the ratio between the current flow in'the reverse direction and the cur.- rent flow in the forward direction upon the appllcation of a direct current voltage to the plate or disc in the reverse and forwarddirections.

A rectified direct current voltage of 3 volts was applied to the individual plates of group B in the reverse direction and in the forward direc tion and the current fiow measured. The .rec- ,tification ratio for the seriesoi standard or control plates of group By averaged 1:640. The rectification ratio for the series of plates of group B made in accordance with the present invention, averaged l 1230.

A third group of plates were made inapproximately the same manner. prepared using a poor grade selenium containing about 0.1% iron cesium bromide. One series of These plates were standard plates.

one volt was applied to the individual plates of each series in the reverse and in the forward di rections and the current flow measured. The

rectification ratio plates average 1:

the standard or control The rectification ratio for for the plates prepared in accordance with this invention average 1:960.

The foregoing provements resulting The improvements in the operating are quite apparent from the vention. characteristics It is obvious that data is representative of the imfrom the practice of my indata. the numerical. values of the leakage current and the rectification ratios will differ with different size rectifier plates, with different samples of selenium,

with different applied voltages and with different additive agents contained in the selenium.

I claim:

1. The method of producing blocking layer devices which comprises providing a supporting electrode vvith a layer or gray,

subjecting the surface of nium,

to the action of vapors tasslum thiocyanate in its decomposition crystalline selethe selenium layer formed by heating po the presence or air to temperature and applying a counter-electrode over the treated selenium surce. 2. The method of producing blocking layer devices Which comprises pp y ng a layer a selenium to a supporting electrode, transforming the selenium into its gray,

ing the surface of tion of vapors cyanate in the temperature and,

crystalline form, subjectthe selenium layer to the ac-' formed by heating potassium thicpresence of air to its decomposition applying a counter-electrode over the treated selenium Surface.

3. The method of producing blocking layer-devices which comprises applying a layer a selenium to a supporting electrode, transforming the selenium into its gray,

ing the surface of crystalline form, EXP'UQ" the selenium layer for from about five seconds to about fifteen seconds to the vapors formed by in the presence of heating potassium thiocyanate air to its decomposition tensperature and applying a counter-electrode over the treated selenium surface.

4. The method 0 f producing selenium rectifier plates which. comprises providing a supporting electrode with a layer nium, subjecting the layer to the action of vapors of gray. crystalline salesurface of the selenium formed by heatin potassium thiocyanate in. the presence oi air to its decomposition t ter-electrode over and subjecting the forming operation.

emperature, applying a counthe treated selenium surface composite unit to an electrical 7 about fifteen second five secorirls'to at 5. The method of producin selenium rectifier plates which comprises applying a layer of selenium to a supporting electrode, transforming the selenium into its gray, crystalline form, passing the coated plate through the vapors formed by heating potassium thiocyanate in the presence of air to its decomposition temperature, applying a counter-electrode over the treated selenium surface and subjecting the composite unit to an electrical forming operation.

6. In a method of producing blocking layer devices including a selenium layer, the step which comprises exposing the surface of the selenium layer to vapors formed by heating potassium thiocyanate in the presence of air to its decomposition temperature.

7. in a method of producing selenium rectifier plates, the step which comprises subjecting the surface or the sele to the action of vapors formed by heating assium thiocyanate in the presence of air to its decomposition temperature.

8. In a method ol' du ing selenium rectifier plates, the step Whi. comprises subjecting the selenium surface om shout five seconds to s .o action of vapors formed icy heating potassium thiocyanate in the presence of air to its decomposition temperature. 9. The oi. :iorn an artificial blockins layer on blocl layer devices including a of seleruum i comprises subjecting the surface of he sel to the action of vapor formed hy heati potassium thiocyanate in the presence or air to ecornposition temperature.

ill. The method o i ing an artificial blocking layer on the so: or the selenium layer in a selenium jecting the surface l elenium for from about .riteen. seconds to the acheating potassium thiooi air to its decomposition oi vapors in cyanate in the p1. tion temperature.

' STANLEY S. FRY.

The following file of this pat N JEEi CETED Date Jan. '7, 1941 Nov. 14, 1944 ERENCES yanogen Gompounds and illstunation, thy Herbert e A. Churchill, 1915.

Eli. i1 M a lln y Elly.

.e which comprises sub I 

